1. Field of the Invention
The invention relates to a lapping device for a safety relief valve, more particularly to a planetary-type lapping device for finish-grinding the valve seat of a safety relief valve.
2. Description of the Related Art
A safety relief valve is usually installed in a container, such as a boiler or a compressed gas receiver and is set to open at a predetermined pressure so as to avoid bursting of the container. A high degree of precision is required when manufacturing or repairing the safety relief valve so as to prevent the leakage of fluids, such as steam or gas, from the container. Because of the high degree of precision which is required when manufacturing or repairing the safety relief valve, the safety relief valve therefore has a relatively high manufacturing cost. Prolonged use of the safety relief valve usually results in a rough, uneven surface at the valve seat of the safety relief valve. Since replacement of the safety relief valve entails a large additional cost, the surface of the valve seat is usually finish-ground in order to prolong the useful life of the same.
FIG. 1 is an illustration of a conventional lapping device (1) for finish-grinding the valve seat (92) of a conventional safety relief valve (90). The lapping device (1) includes an upwardly extending handle (11), a disc member (14) provided on a lower end of the handle (11) and a positioning rod (12) which extends downwardly from the disc member (14). Grinding projections (13) are provided on a lower side of the disc member (14).
The positioning rod (12) is extended into a valve opening (91) that is defined by the valve seat (92) when the lapping device (1) is used to finish-grind the surface of the valve seat (92). The grinding projections (13) are in contact with the valve seat (92) at this stage. The handle (11) is then rotated manually, thereby permitting the grinding projections (13) to lap the surface of the valve seat (92).
The disadvantages of using the lapping device (1) for finish-grinding the valve seat (92) are as follows:
1. The lapping device (1) is manually operated, requires the application of a relatively large force and further requires a relatively large operating space so as to facilitate proper operation the same.
2. Since the lapping device (1) is rotated about an axis which is coaxial with the axis of the valve seat (92), frequent finish-grinding of the surface of the valve seat (92) can result in the formation of annular grinding grooves thereat. These grinding grooves can affect the precision of the safety relief valve.
3. Since the lapping device (1) is manually operated and since the force which is exerted when the lapping device (1) is operated is not constant, the surface of the valve seat (92) remains uneven and may be slanted when finish-ground. This can result in the untimely release of high pressure fluids.